1. Field of Use
This invention relates to the purification of caustic liquors and high caustic streams. More particularly, the invention relates to the removal of iron and other colorants, especially anionic contaminants, from a sodium aluminate (or Bayer) liquor. The invention further relates to a means for producing: an improved caustic (NaOH); and improved aluminum hydroxide product having higher whiteness/brightness levels.
2. Technology Review
The recovery of aluminum hydroxide from bauxite ore and similar alumina-bearing materials according to the Bayer process is achieved by digesting hydroxide-containing ore with a caustic liquor. A major portion of alumina is dissolved by this liquor, while most unwanted ore constituents remain undissolved making them separable from the liquor. Such undissolved constituents are sometimes referred to as "red mud". After pressure digestion of bauxite with a caustic liquor such as sodium aluminate, the red mud may be removed from this liquor by decantation and filtration. Aluminum hydroxide is then separated from a supersaturated sodium aluminate liquor, also known as "green" or "pregnant" liquor, typically by precipitation. During such precipitation, the supersaturated sodium aluminate liquor is cooled and mixed with a slurry of fine aluminum hydroxide particles which act as seeds to induce formation of more aluminum hydroxide. Following such precipitation, this slurry is pumped through a classification system through which a coarse fraction of crystallized aluminum hydroxide is separated from the liquor while the resulting spent sodium aluminate is recycled to the digester for mixing with new (incoming) bauxite.
High levels of impurities are undesirable in the Bayer (or "green") liquor used to produce aluminum hydroxide. Because these impurities decrease the whiteness or color purity of the aluminum hydroxide precipitated therefrom, it is desirable to minimize their presence in green sodium aluminate before any crystallization takes place. Typically, the impurities result in an aluminum hydroxide product having lower than about 70% whiteness. Such impurity levels impair the use of this hydroxide in commercial products where a high level of whiteness is required. The present invention solves the problem of poor whiteness while producing an aluminum hydroxide having whiteness or brightness levels of about 80% or more (based on a 100% TiO.sub.2 reference standard).
When organic and inorganic impurities are present in a caustic liquor, process problems such as lower liquor productivity and reduced alumina purity also result. Still other difficulties caused by the presence of organic impurities include: lower alumina yield; the generation of excessive fine hydroxide particles; production of a colored liquor and colored aluminum hydroxide; lower red mud settling rates; the loss of caustic due to formation of sodium organics; an increase in liquor density; increased viscosities; a raised boiling point; and unwanted foaming of the liquor.
Numerous methods are known for controlling and/or removing organics from a Bayer process (or green) liquor. These include treatment of the process liquor with sodium hypochlorite or other oxidizing agents such as oxygen or air. For example, German Patent No. 2,945,152 describes a process for removing organic compounds from Bayer liquors by heating to 120.degree.-350.degree. C. and introducing oxygen-containing gas until a partial pressure of 3-30 atmospheres is reached. Australian Patent No. 12085/83 discloses treating Bayer liquors with reactive magnesium oxide or magnesium hydroxide before calcining the resulting magnesium-aluminum compound at 900.degree. C. or more. In Japanese Patent No. 59-102819, organic colorants are removed from an aluminate solution through contact with a waste salt mud consisting of 30-60% calcium sulfate, 10-70% calcium carbonate and 5-20% magnesium hydroxide. Inao et al U.S. Pat. No. 4,215,094 discloses an organic removing process which includes contacting sodium aluminate solutions with molecular oxygen containing gases in the presence of copper ions (catalyst) at elevated temperatures of from 180.degree. to 300 .degree. C. According to Swinkels et al U.S. Pat. No. 4,836,990, organics may be removed from Bayer liquors through contact with manganese dioxide. Such contact causes the organics therein to oxidize.
It is known to remove still other impurities from Bayer liquors by treatment with alkaline earth compounds. Schepers et al U.S. Pat. No. 4,046,855, for example, teaches treating aluminate liquors with a magnesium compound to remove organic colorants. Mercier et al U.S. Pat. No. 4,101,629 treats Bayer process solutions with a barium compound to remove impurities. German Patent No. 2,415,872 adds a calcium compound to the process liquor to remove humic matter as insoluble calcium compounds.
In The et al U.S. Pat. No. 4,676,959, an aluminum hydroxide product having improved levels of whiteness is produced by passing caustic solutions through specially coated, hollow sulfonated polysulfone fibers. In The et al U.S. Pat. No. 4,678,477, a method for purifying caustic solutions is disclosed which includes contact with the outside of specially coated, polysulfone fibers and collection of purified product from an open end of these fibers.
The impurity levels of organics, such as sodium oxalate, in sodium aluminate solutions have also been lowered using cationic sequestrants comprising quaternary nitrogen compounds with medium and long chain alkyl groups as taught in Lever U.S. Pat. No. 4,275,042. DeLaBretique U.S. Pat. No. 3,457,032 discloses purification of a strongly alkaline solution using strongly basic and macroreticular anion exchange resins.
In Carruthers et al U.S. Pat. No. 4,038,039, the removal of sodium oxalate from a sodium aluminate liquor by spraying concentrated liquor onto a packing material is disclosed. Bush et al U.S. Pat. No. 4,496,524 teaches removing sodium oxalate from spent Bayer liquors with ethanol. Such treatment causes the sodium oxalate to precipitate from the liquor.
Yamada et al U.S. Pat. No. 4,280,987 removes carbon compounds from a Bayer liquor stream by adjusting the molar ratio of aluminum component to sodium component before heating the liquor to: (i) form sodium aluminate; and (ii) drive the carbon compounds off as carbon dioxide. Bird et al U.S. Pat. No. 4,282,191 describes removing zinc impurities from sodium aluminate caustic using zinc sulfide as seed material.
Columbo et al U.S. Pat. No. 3,295,961 discloses a process for removing iron impurities from the red mud slurry of a Bayer process by first drying the red mud before heating to reduce the iron compounds therein to metallic iron for subsequent separation from dried red mud using magnetic separators. Goheen U.S. Pat. No. 3,729,542 teaches removing iron impurities in a sodium aluminate solution by filtering through a bed of iron particulates. Japanese Patent No. 56-18534 separates iron from a sodium aluminate stream using preheated red mud or bauxite.
Japanese Patent No. 57-31527 discloses a method for producing aluminum hydroxide of high whiteness by adding one or more types of alkaline earth metal compounds to a sodium aluminate (Bayer) solution. Representative compound additives include oxides, hydroxides, carbonates, silicates and nitrates of magnesium or calcium; and carbonates, silicates, nitrates and sulfates of barium.
In Tsai U.S. Pat. No. 3,574,537, iron oxide is extracted from red mud by passing SO.sub.2 through a solution of the mud. Still other impurities such as SiO.sub.2 precipitate out upon heating until a pH of 4.5-5.0 is attained. Adams U.S. Pat. No. 3,796,789 discloses lowering the iron content of sodium aluminate liquors by bringing the liquor to atmospheric conditions, adding slaked lime thereto, and subsequently digesting the same. Dobos et al U.S. Pat. No. 3,989,513 treats Bayer process red mud to obtain raw iron and a slag from which sodium aluminate and calcium aluminate can be leached. German Pat. No. 3,501,350 adds a mixture of calcium oxide/hydroxide and kierserite (MgSO.sub.4 .multidot.H.sub.2 O) to Bayer liquors for lowering the impurity levels (especially Fe content) of the resulting aluminum hydroxide. In Russian Patent No. 468,888, sodium sulfate is added to increase the efficiency of precipitating a lower iron hydroxide from an alkaline aluminate solution.
Green et al U.S. Pat. No. 4,083,925 precipitates ferrous iron from an alkali metal aluminate liquor using an anionic polyacrylamide having a molecular weight of at least 2,800,000. Owen et al U.S. Pat. No. 4,713,222 shows removing iron compounds from Bayer liquors by seeding with ferrous ammonium sulfate before adding a starch and/or acrylic acid polymeric flocculant thereto. In Spitzer et al U.S. Pat. No. 4,717,550, the iron content of a Bayer process stream is reduced using a tertiary hydroxyl-containing polyamine.
McDaniel U.S. Pat. No. 4,324,769 claims an improved process for extracting alumina hydrate from ore, said hydrate having an iron content of less than 0.03% by weight. The process involves digestion with various alumina-to-caustic ratios in a series of separate reaction vessels maintained at different temperatures. In McDaniel U.S. Pat. No. 4,446,117, a variation of the aforementioned process produces alumina hydrate containing less than 0.017% iron oxide by weight.
Various methods for using hydrotalcite-like compounds are also known. Anabuki et al U.S. Pat. No. 4,415,555, for example, discloses treating an iron deficiency disease with a hydrotalcite-like structure, one embodiment of which has the formula: EQU (Fe.sup.2+).sub.x (Al.sup.3+).sub.2 (OH).sub.2x-6-2z (CO.sub.3.sup.2-).sub.z .multidot.mH.sub.2 O,
wherein 1.ltoreq.x .ltoreq.20, 0&lt;z&lt;3 and 0.ltoreq.m.ltoreq.8. A related compound for increasing the iron level of hemoglobins and/or serums is claimed in Miyata et al U.S. Pat. No. 4,629,626. In Sood U.S. Pat. No. 4,752,397, heavy metal ions are removed from an impure aqueous solution using activated hydrotalcite. Representative metal ions removed by this process include: antimony, arsenic, beryllium, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, thallium, tin and zinc.